1. Field of the Invention
The present invention relates to a measuring device for measuring at least one property of a material web, in particular a paper and/or board web.
2. Description of the Related Art
Various paper quality sensors are known, in which the measurement is normally carried out over a measuring gap whose thickness is several millimeters. Examples which may be mentioned of sensors which are relevant to the present invention are:                gloss sensor,        formation sensor,        surface roughness sensor,        grammage and ash sensors (sensors with radioactive sources),        ash sensors for measuring a respective composition by way of x-ray fluorescence,        grammage sensor based on an x-ray tube for measuring the total ash content and the ash composition.        
Most of the sensors mentioned above can certainly currently be obtained as such. However, they require an extremely stable and therefore costly scanning mechanism in order to maintain the alignment of the sensors when the latter are moved over the sheet. What is needed in the art is a device where the requirements on accuracy for the scanning mechanism are reduced considerably.